In a known procedure which makes use of the mud pressure pulsing principle, changes in pressure are made by means of a temporally limited, alternating increase in the pressure of the flushing liquid from a predetermined initial pressure to a predetermined maximum pressure and subsequent reduction of the flushing liquid pressure back to the initial pressure. Thereby the normal working pressure of the flushing liquid is the original starting pressure in the transmission region. Such a series of "positive" pressure pulses are achieved in practice with the aid of a valve in the transmission region of the drill string which reduces and subsequently increases the flow cross-section in the drill string by means of closing and opening its valve body. The pressure difference between maximum and starting pressures, i.e., the amplitude of a pressure-time curve of the pressure pulse series, which can be achieved by this means is very limited and is low in comparison to the value of the starting pressure. Furthermore, the damping effect of the drill string surrounding the transmission medium on the amplitude of the pressure-time curve increases with increasing speed of such pressure change, i.e., with increasing frequency of the pressure-time curve of the pulse series. Hence, pressure pulses of this kind, after passage of the transmitting medium through instrument transformers on the earth surface which measure the pressure in the flushing medium and are designed to reconvert the received pressure pulse series into evaluable informational data, are clearly recognizable only with difficulty, because they are superimposed and distorted by pressure variations in the flushing liquid, which result from numerous influencing factors of the boring procedure itself. An increase in the amplitude of the pressure to desirably large magnitudes is not possible due to the detrimental effect on the boring operation and due to the strong increase in wear phenomena.
In another known process of the kind described, the pressure variation is brought about by a temporally limited, alternating reduction of the pressure of the flushing liquid from an initial pressure to a minimum pressure and increase of the pressure of the flushing liquid from the minimum pressure to the initial pressure. Herewith, also, the normal working pressure of the flushing liquid in the transmission region of the drill string serves as the original pressure and a direct flow connection is established between the drill string and the annular volume of the bore hole which surrounds it to decrease the original pressure to the predetermined minimum pressure. By means of this type of valve regulated circuit, pressure differences between the original pressure and the minimum pressure may be achieved which correspond to the pressure drop of the flushing liquid on its path from the transmission region of the drill string to the rotary drill tool and upward into the annular space. The same difficulties basically hold true in the ability to distinguish "negative" pressure pulse series as mentioned in the beginning in connection with the transmission of "positive" pressure pulse series. An increase in the amplitude of "negative" pressure pulses to desirably large magnitudes does not, to be sure, lead to a strong increase in wear phenomena to the extent as is the case in the generation of "positive" pressure pulses; in respect to its maximum value, it is, however, limited to an amount less than the value of the pressure drop across the rotary drill tool.
The use of either "positive" or "negative" pressure pulses for distance transmission limits the transfer of information due to low transmission speed and limited capability for encoding which is particularly disadvantageous in the transmission of multiple pieces of informa- tion.
To counter this, a further known process of production of "positive" and "negative" pressure pulses of the type described in the overall concept of claim 1 (U.S. Pat. No. 4,027,282) is employed. In this method, the original pressure starts out as an increased pressure compared to the normal working pressure of the flushing liquid in the transmission region of the drill string, which is then raised to a predetermined maximum pressure or lowered to a predetermined minimum pressure, where the predetermined minimum pressure corresponds to the normal working pressure of the flushing liquid in the transmission region of the drill string. This process widens the possibilities for encoding, to be sure, and increases the transmission velocity, however, the recognizability of the pressure pulses received at the surface of the earth is reduced thereby compared to processes using pure "positive" or "negative" pressure pulses, because the total pressure difference between the maximum pressure and the minimum pressure does not exceed the value which, because of the above-named reasons, are achievable in the production of "positive" pressure pulses only. It must also be considered that an apparatus suited to perform this process is expensive to build, complicated and susceptible to trouble, and that a valve which is closed for a long period of time to achieve the increased original pressure is quickly destroyed or damaged by erosion. This applies, as well, to a partially closed valve to achieve this purpose (U.S. Pat. No. 2,759,143). The task underlying the invention is to create a process of the kind described by which pressure pulses with significantly higher pressure differences at an increased transmission speed can be produced, wherewith pressure pulse series with a large extent of configurational variation are capable of being prepared. At the same time, interference with (injury to) the boring operation and the boring apparatus itself is significantly reduced.